To provide an illumination device in which there is no variation in light intensity distribution of the illumination device, and in which illumination spots, particularly light intensity spots, are not prone to occur in a read image in an image reading device, by maintaining a constant gap between a light source and an end face of a light guide. The illumination device includes: a light guide having an end face for taking in light, a diffuse reflecting surface for diffusely reflecting the light taken in from the end face, and a light exit surface for emitting the light that is diffusely reflected at the diffuse reflecting surface towards an irradiation surface. The illumination device further include a reflector having a diffuse reflecting surface that reflects light from the light source toward the one end face of the light guide. The light guide has, at the one end, a flange portion that abuts the reflector, the light source is mounted to a circuit board, and the reflector is held between the flange portion of the light guide and circuit board so as to maintain a predetermined gap between the light source and light guide.